338 U. 8. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



ARCTOMYS, Schreber.i 



Arctomyt, "ScHRKBEB, Saugthiere, IV, 1792." 



AOD. & BACH. N. Am. Quad. I, 1849, 16. 



g p Q H gjjjg ] ar g e ; body very thick and depressed ; thumb very rudimentary, armed with a small flat nail, not a claw ; 



eoles entirely naked ; cheek pouches very shallow ; pupil rounded? 



It is in this genus that we find the largest members of the Sciurinae, some of them, indeed, 

 taking the second or third rank in size among North American rodents, being exceeded only by 

 Castor and Hystrix, and one species of hare, the body is thick and massive, the ears distinct 

 and well shown above the fur, though short and rounded. The tail is bushy and full, but nearly 

 cylindrical, the hairs being as long above and below as laterally. The thumb is rudimentary, 

 with a short flat nail, not a claw. The outer finger of the hand extends as far as two-thirds 

 the length of the palm, and in all the feet the claw of the outer toe reaches beyond the base of 

 the one next to it. 



The skull is much more flattened than in the spermophiles, in fact its upper outline is not 

 only nearly straight, but it is decidedly concave between the orbits. The post-orbital processes 

 are very large, the distance between their apices equal to the greatest width of the back part of 

 the head ; their posterior margin is rectilinear and perpendicular to the axial line of the skull, 

 until it passes gently into the temporal ridge of the parietal bone, which is quite prominent 

 and concave externally. There is a distinct semicircular notch behind the subtubercular ante- 

 orbital process of the frontal ; the nasal bones extend back nearly as far as this notch. The 

 malar bone is twisted angularly, and shows externally three principal faces ; anteriorly, it is 

 wedged between the maxillary and the lachrymal bones. The palate is broad and rather plane, 

 though flattened concave between the molars the bottom of the palate on a level with the in 

 ferior edge of the root of the zygoma. The external outlines of the upper molars are rectilinear 

 and parallel in the adult curved in the young ; the inner outlines are slightly divergent ante 

 riorly in the joung. The plane of the grinding surface is nearly horizontal at first, but twisted 

 outwards more and more obliquely behind. There is no very essential difference in the shape 

 of the lower jaw of Sciurus and Arctomys. 



The incisors are much less compressed than in Sciurus, considerably rounded anteriorly and 

 laterally ; the enamel extends half way round the side. There are faint indications of numer 

 ous fine strife on the enamel. The crown of the anterior upper molar is about half the area of 

 the second, and its summit is on the same level with the rest. The other teeth are nearly equal, 

 but increase gradually posteriorly. The first molar has one root, the others three, each ; the 

 crown of the first molar, in its most perfect state, exhibits a semicircular marginal ridge on the 

 inner, anterior, and posterior sides, thus encircling nearly three-fourths of the crown ; within 

 this curve rises a transversely elongated tubercle. 



In the young skull the deciduous first upper molar is much smaller than its successor ; the 

 crown however, is not a simple conical tubercle, but shows a distinct longitudinal ridge, with a 

 pit on either side ; in this case, however, the direction of the ridge is nearly with the line of 



1 This is the authority given by Agassiz in Nomenclator Zoologicus. I find the name used, however, by Gmelin, in 1788. 

 It Is probable that Schreber made it in one of the earlier numbers of his volume IV, which was not completed nor furnished 

 with a title until 1792. 



